1. Field of the Invention
The present invention generally relates to methods for measurement or analysis of a nitrogen concentration of a specimen. Certain embodiments relate to methods that include determining a nitrogen concentration of a nitrided oxide gate dielectric formed on a specimen from spectroscopic ellipsometric data.
2. Description of the Related Art
Optical systems play a significant role in the manufacturing of integrated circuits and other semiconductor devices. For example, optical lithography tools are used to transfer a pattern from a reticle to a resist coated wafer. The patterned features on the wafer can then be used to form various features of integrated circuits and semiconductor devices. In addition, optical metrology and/or inspection tools are used for quality control purposes in semiconductor manufacturing. The capability and throughput of these optical systems can have a significant impact on semiconductor manufacturing. For example, the throughput of an optical lithography or metrology and/or inspection tool has a direct impact on the throughput of a semiconductor manufacturing process (e.g., as the throughput of the tool decreases, the throughput of the process decreases). In addition, the resolution of a lithography tool often determines the lateral dimensions of features of an integrated circuit. Therefore, the resolution of the lithography tool can determine other characteristics of the integrated circuit such as the performance bin characteristics. Likewise, the resolution capability of an optical metrology and/or inspection tool can have a significant impact on a semiconductor manufacturing process since the accuracy of the optical metrology and/or inspection tool can directly affect how well the process is controlled.
The resolution of an optical system depends to a large extent on the wavelength of the optical system as well as other parameters such as numerical aperture (NA). For example, as the wavelength of the optical system is decreased, the optical system can image features having smaller and smaller dimensions thereby increasing the resolution of the system. Decreasing the wavelength of a system such as a lithography tool is one way for semiconductor manufacturers to image features onto a wafer having smaller dimensions. Many lithography tools used in semiconductor manufacturing today are designed for use with light having a wavelength of 248 nm. However, lithography tools that are designed for use with light having a wavelength of 193 nm are becoming more prevalent in semiconductor research and manufacturing.
At wavelengths around 193 nm, light may be partially absorbed by water, oxygen, and air that is present in the optical path of an optical system. However, absorption levels at these wavelengths are not generally problematic. In contrast, as the wavelength of optical systems falls below 190 nm, absorption of the light by water, oxygen, and air can cause significant problems for these systems. For instance, in lithography tools designed for use at 157 nm, the amount of light available for imaging a resist on a wafer may be insufficient due to the absorption of the light by air in the lithography tool. Furthermore, many of the light sources that are able to produce light at wavelengths less than 190 nm are relatively low intensity or power light sources. Therefore, any absorption of the light by the environment in the lithography tool can result in a severe reduction in the imaging capability of the optical system.
To reduce the amount of light that is lost to absorption by air, some systems can be designed to generate a vacuum in which the optical components of the systems and the specimen that is being imaged can be placed. Since generating and maintaining a vacuum can be relatively expensive, however, more common methods for reducing absorption of light having wavelengths less than 190 nm involve purging the housing in which the optical components and the specimen are placed. Purging the housing or the tool generally involves replacing the ambient environment within the housing or tool with relatively pure gas such as nitrogen. There are, however, several problems with the current methods that are used to purge optical systems. For example, currently used methods of purging generally involve purging a relatively large region of the tool (e.g., the entire tool or the entire measurement chamber). In addition, purging a large region of the tool takes a significant amount of time. Therefore, purging can have a significant adverse impact on the throughput of the optical system.
Accordingly, it would be advantageous to develop systems and methods for optical tools that are designed to use light that is at least partially absorbed by air and that have more efficient purging systems than those described above.